When performing a multiphase core flooding experiment, the quality of fluids' preparation is of great importance. Unlike single-phase core flood, to measure relative permeability of each fluid it's crucial to have all fluids in thermodynamic equilibrium, which is determined by pressure and temperature stabilization. The situation becomes critical when a fluid pair is gas and its condensate—gas and liquid phases of very similar physical properties. If the phases are not in the thermodynamic equilibrium, then decrease in volume of one phase takes place along with increase in volume of other phases. The latter circumstance makes it practically impossible to pump each of phases in a monitored manner at a fixed rate of volume flow.
Usually, a fluid pair gas-condensate is prepared in either a thermodynamic cell (a PVT-cell, a PVT bomb) or in a special container (a tank) at certain specified pressure and temperature. The thermodynamic equilibrium for fixed pressure and temperature is considered to be achieved after pressure stabilization of phases. For various reasons, it may be convenient to separate phases (liquid and gas) into different containers for experimental studies.
The need for transportation and storage of containers with separated phases often occurs, for example, for movement from a fluid thermodynamic study laboratory to a core flooding (filtration) laboratory. In this case, the thermodynamic disequilibrium is likely to occur due to variations of external conditions (temperature, pressure, mechanical effect). Even after a long time after the temperature stabilization in the containers, the phases after mixing thereof will most probably come out of the thermodynamic equilibrium state and will not achieve the initial state (which was after preparation of the equilibrium gas condensate mixture). As a result, the fluid pair injected into the core can be out of thermodynamic equilibrium.
In case of gas-condensate pair, this effect is expressed in occurrence of phase transitions: from a gas into a liquid and vice versa. As a consequence, a volumetric phase ratio specified in injection pumps may significantly differ from a volumetric phase ratio for a fluid before entering a core. In practice, it is suggested that fluids entering a core are already in the thermodynamic equilibrium which, as a rule, is not checked.
All existing experiments for filtration of a gas condensate mixture through cores can be divided into two groups: injecting a mixture in a single-phase (gaseous) state followed by separation into gas and liquid phases and by accumulation of the liquid phase (a gas condensate) in the core due to a pressure drop caused by filtration; and separate injection of both—gas and liquid—phases simultaneously with different volumetric fractions in a flow into a core. If a goal of the filtration experiment is to determine phase permeabilities, then, the most reasonable option is simultaneous injection of both—gas and liquid—phases that are in thermodynamic equilibrium.
Some researchers (cf., H. Calisgan and S. Akin, Near Critical Gas Condensate Relative Permeability of Carbonates, The Open Petroleum Engineering Journal, 1, 30-41 1874-8341/08, 2008, Bentham) proceed as follows: a pore space of a core is saturated with a methanol-enriched liquid phase prior to each test, said phase dissolving any residual amount of n-hexane. A temperature of a filtration system is set according to requirements of the experiment. The methanol-enriched liquid phase goes into the thermodynamic equilibrium with the n-hexane phase. Finally, the n-hexane-enriched phase is injected into the core at a required volumetric flow rate. In this case, however, the researchers don't check the presence of the thermodynamic equilibrium between the phases.
It is known the method for checking thermodynamic equilibrium of gas and condensate, said method comprising steps of using a transparent thermodynamic (PVT) cell and visual observing an interface between gas and liquid phases. This system is described in H. L. Chen, S. D. Wilson, and T. G. Monger-McClure. 1999, Determination of Relative Permeability and Recovery for North Sea Gas-Condensate Reservoirs. SPE Reservoir Eval. & Eng. 2 (4), August 1999.
Many researchers prepare gas and condensate separately prior to the experiment and place containers with phases into a filtration system supposing that both phases are at thermodynamic equilibrium (see, for example, M. Jamiolahmady, M. Sohraby, S. Ireland. 2008, Gas condensate relative permeabilities in propped porous media: coupling versus inertia. SPE Annual Technical Conference and Exhibition, SPE 115726).